Parasitism

Definition

A symbiotic interaction in which one organism (the parasite) lives on or in another organism (the host), deriving resources or benefits while causing measurable harm to the host, typically without killing it immediately. Distinguished from predation by the sustained, intimate association with a living host and from commensalism by the negative fitness impact on the host. In arthropods, parasitism encompasses both ectoparasites (e.g., lice, fleas, varroa mites) that feed externally and endoparasites (e.g., parasitoid wasp larvae developing within caterpillars) that occupy host tissues or body cavities.

Etymology

From Greek parasitos, "one who eats at another's table" (para-, "beside" + sitos, "grain, food"), via Latin parasitus.

Example

The varroa mite (Varroa destructor) practices parasitism on honey bees: the mite enters the hive on foraging workers, reproduces in brood cells, and feeds on hemolymph, weakening individual bees and vectoring viral pathogens that contribute to colony collapse.

Related Terms

• parasitoid
• ectoparasite
• endoparasite
• host
• symbiosis
• Commensalism
• Mutualism
• Brood parasitism
• Hyperparasitism
• parasitic castration

Usage Notes

Parasitism is sometimes used broadly to include parasitoidism, but specialists often distinguish them: true parasites do not invariably kill the host, whereas parasitoids typically do. The harm caused ranges from subtle fitness reductions (reduced fecundity, slower growth) to severe pathology. Some authors reserve "parasitism" for cases where the parasite is smaller than the host and has high host specificity; others apply it more widely. In medical and veterinary entomology, the term frequently implies arthropod vectors that transmit disease agents while feeding.